A new drug is undergoing testing, at 10am 100mg is given to a patient. At 10pm a blood sample is taken from the patient and the patient is found to have 12.5mg of the drug circulating.

Which of the following best describes the half-life of this drug?

A) 2 hours
B) 4 hours
C) 6 hours
D) 8 hours
E) 10 hours


The answer is B. A half-life is the time taken for half of the initial amount to be metabolised, in this instance we have gone from 100mg to 12.5mg in 12 hours - this represents 3 half-lives [100 --> 50 --> 25 -->12.5] in 12 hours, or one half-life every 4 hours.